The present invention relates to electric power units, and more particularly to an electric power unit in which, when the AC input voltage is changed from one value to another, switching is automatically effected between a voltage-multiplying rectification function and a nonvoltage-multiplying rectification function.
Some electronic devices are designed so as to operate on various different AC input voltages, such as 100 V and 220 V. In such an electronic device, in the event that the AC input voltage to the electric power unit is switched over to another value. the DC output must be maintained unchanged. In order to meet this requirement, a method may be employed in which, in the power unit, a DC output is produced using ordinary rectification when a high AC input voltage is present, and, in the case of a low input voltage, the DC output voltage is increased by voltage-multiplying rectification. In the case where the electric power unit has two different rectification functions as described above, provision for switching between the two functions is essential. For this purpose, mechanical switching may be employed. However, in this case, it is considerably troublesome to effect the switching operation, which must be carried out manually. If the switching operation is not performed properly, for instance, if voltage-multiplying rectification is employed in the presence of a high AC input voltage, an overly high DC output will be produced, damaging the electronic device powered by the power unit.
Electronic control can be employed for switching the rectification functions, in which case the switching operation can be automatically achieved by detecting the input voltage. However, the speed at which such control can be carried out is greatly affected by the speed at which a voltage for driving the switching control device can be established. That is, if the voltage for powering the switching control device is established with a delay, the reliability in switching the rectification functions may be low.
Accordingly, an object of the invention is to provide an electric power unit in which, in response to the switching of an input voltage to another value, the aforementioned rectification functions are automatically switched quickly and with high reliability.